Step motors (sometimes called stepper motors) are electromagnetic motors which are designed such that they have a large number of preferred rest positions, that is; the internal magnetic torques in the motor urge the rotating shaft to one of several specific locations. Step motors are extremely useful in any application which requires incremental motion. They are particularly important in the data processing industry since they are ideally suited for positioning the heads on disk drives, positioning rotating print devices such as daisy wheels or thimbles and for use in many other devices requiring fast and accurate positioning of rotary or sliding elements at a low cost.
An excellent prior art description of the various types of step motors available and their operation is contained in a publication entitled Warner Electric's Guide to Selecting and Controlling Step Motors, copyright 1979, by the Warner Electric Brake and Clutch Company of Beloit, Wis. (hereinafter "Warner"). This publication provides a source for terminology and explanation of associated but noncritical elements of the motors and circuitry utilized with the present invention.
As is described in part III. L of Warner, one of the primary necessary associated considerations in dealing with step motors is in providing a method of damping the oscillation which results when a motor element having rotational momentum is caused to stop at a particular point. It is an inevitable consequence of all step motors that the rotation momentum results in an overshoot and a certain degree of oscillation will occur whenever it is necessary to stop the rotation of the shaft at a desired rest point. The magnitude of this overshoot oscillation is ordinarily in the range of 15 to 100 milliseconds (ms) or more. Consequently, it has been a continuing goal of prior art devices to minimize the oscillation about the desired rest point and thus to minimize the time required to bring the motor and the associated device to a full stop.
Newer applications have required higher speed motors and have thus also required better, more powerful and quicker oscillation damping mechanisms. It is also desirable that the damping mechanisms not have deleterious effects on the other characteristics of the motor such as the high speed resistance torque, the pull-out torque, the pull-in torque, the holding torque, the positioning accuracy and other relevant characteristics. Additionally, due to a high degree of competition, the desired methods cannot add drastically to the cost involved.
As outlined in part III. L of Warner, damping mechanisms and schemes come in a great variety.
Some prior art examples of stepper motors and damping apparatus and schemes that have been memorialized in United States patents are U.S. Pat. No. 3,466,476 issued to Snowden, U.S. Pat. No. 3,621,312 issued to Palmero and U.S. Pat. No. 4,070,592 issued to Snowden. These references show hybrid type step motors and refer to the importance of achieving rapid and effective damping.
One common method of achieving damping in step motors is in the use of electrical circuitry to produce feedback signals. A prime example of this is found in U.S. Pat. No. 3,465,225 issued to O'Regan.
Another common method for achieving oscillation damping is to use a fluidic camping mechanism on the rotating shaft elements. Some examples of the use of this type of damping are found in U.S. Pat. No. 3,286,109, issued to Madsen and U.S. Pat. No. 3,890,514 issued to Mutz. One disadvantage of the use of fluidic damping on the shaft mechanisms is that the high speed torque resistance of the motor is increased by the effective momentum friction of the fluidic medium. This results in a reduced high-speed torque for the motor since the drag torque of the damping scheme increases with the velocity of rotation.
Another common category of damping mechanisms occurs with the use of mechanical resistance dampers. Some examples of these type of dampers are found in U.S. Pat. No. 3,197,659, issued to Marshall, U.S. Pat. No. 3,453,465, issued to De Boer, U.S. Pat. No. 3,496,393, issued to Reifman and U.S. Pat. No. 3,790,831, issued to Morreale. The use of a mechanical damper can be very effective but it does require a greater amount of torque to be applied to the motor shaft in order to overcome the load inertia of the damping mechanism. This requires a more powerful motor which increases the cost.
As with all areas of technology, there remains significant room for improvement relating to lowering the cost, increasing the efficiency and maximizing the effectiveness of a given mechanism or type of process. This remains true regarding methods of damping step motors.